As a result of the interest in recent years to reduce the emission of pollutants from burners of the type used in large furnaces and boilers, significant improvements have been made in burner design. In the past, burner design improvements were aimed primarily at improving heat distribution to provide more effective heat transfer. However, increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.
Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to, nitrogen oxide and nitrogen dioxide. Reduction of NOx emissions is a desired goal to decrease air pollution and meet government regulations.
The rate at which nitrogen oxide is formed is dependent upon the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone, and (3) excess oxygen supply. The rate of formation of nitrogen oxide increases as flame temperature increases. However, the reaction takes time, and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, over a longer period of time.
One strategy for achieving lower NOx emission levels is to install a NOx reduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, it represents a less desirable alternative to improvements in burner design.
Burners used in large industrial furnaces may use either liquid or gaseous fuel. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion and mix combustion air with the fuel at the zone of combustion.
Gas fired burners can be classified as either pre-mix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.
Raw gas burners inject fuel directly into the air stream, such that the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763. In addition, many raw gas burners produce luminous flames.
Pre-mix burners mix some or all of the fuel with some or all of the combustion air prior to combustion. Since pre-mixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Pre-mixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, pre-mix burners are often compatible with various steam cracking furnace configurations.
Floor-fired pre-mix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a pre-mix burner is the burner of choice for such furnaces. Pre-mix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
One technique for reducing NOx that has become widely accepted in industry is known as combustion staging. With combustion staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature can dramatically reduce NOx emissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase.
In the context of pre-mix burners, the term “primary air” refers to the air pre-mixed with the fuel; “secondary,” and in some cases “tertiary,” air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air are more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 4,629,413 discloses a pre-mix burner that employs combustion staging to reduce NOx emissions. The pre-mix burner of U.S. Pat. No. 4,629,413 lowers NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The entire contents of U.S. Pat. No. 4,629,413 are incorporated herein by reference.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from pre-mix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Air flow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing pre-mix burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
Analysis of burners of the type disclosed in U.S. Pat. No. 5,092,761 has shown that the flue gas recirculation (FGR) ratio is generally in the range of 5 to 10%, where the FGR ratio is defined as:
      FGR    ⁢                  ⁢    ratio    ⁢                  ⁢          (      %      )        =      100    ×                  (                              lb            .                                                  ⁢            of                    ⁢                                          ⁢          flue          ⁢                                          ⁢          gas          ⁢                                          ⁢          drawn          ⁢                                          ⁢          into          ⁢                                          ⁢          venturi                )                                                        (                                                                    lb                    .                                                                                  ⁢                    fuel                                    ⁢                                                                          ⁢                  combusted                  ⁢                                                                          ⁢                  in                  ⁢                                                                          ⁢                  burner                                +                                                                                                                          lb                  .                                                                          ⁢                  air                                ⁢                                                                  ⁢                drawn                ⁢                                                                  ⁢                into                ⁢                                                                  ⁢                burner                            )                                          The ability of existing burners of this type to generate higher FGR ratios is limited by the inspirating capacity of the fuel orifice/gas spud/venturi combination. Although further closing of the primary air dampers can further reduce the pressure in the primary air chamber and thereby enable increased FGR ratios, the resultant reduction of primary air flow is such that insufficient oxygen is present in the venturi for acceptable burner stability.
As disclosed in “The Design of Jet Pumps” by A. E. Knoll, appearing in Vol. 43 of Chemical Engineering Progress, published by the American Institute of Chemical Engineers (1947), it is known to optimize the operation of venturis used in air and steam operated air movers at relatively mild (roughly ambient) temperatures. In contrast, in the burner of the invention, combustible gaseous fuel (including, but not limited to, methane, H2, ethane, and propane) is used to move a combination of very hot (above 1000° F., 540° C.) flue gases, hot air, hot uncombusted fuel, CO, and ambient air.